The hydration behaviour of sixteen ashes, obtained from different commercial-scale fluidized bed combustors, has been investigated. Hydration is important for both ash disposal and reactivation of excess lime present in the ashes for further use in flue gas desulphurization. The techniques used were instrumental and conventional chemical analysis, thermogravimetry and X-ray diffraction. The ashes comprised both fly ash and bottom ash, with particle size less than 2 mm. The ashes were heat treated in air to oxidize free carbon and then hydrated with pressurized steam at about 170 degrees C, alone and with addition of pure CaO. It has been shown that steam hydration is effective in quantitatively converting CaO to Ca(OH)(2), but in most cases the free lime content (i.e. CaO + Ca(OH)(2)), expressed as CaO, decreases and added CaO enters into pozzolanic reactions with coal ash components, in part or even completely. Both the chemical evidence and X-ray phase analyses indicate that hydrated silicates and silicoaluminates are formed. The hydrated ashes are all able to take up additional SO2 and it appears that the presence of amounts of Ca(OH)(2) detectable by phase analysis is not necessary for such capture. Crown Copyright (c) 2005 Published by Elsevier Ltd. All rights reserved.